In FM radio receivers, countermeasures are generally taken against various types of noise so as to improve sound quality. Particularly, in an in-vehicle radio receiver, there are instances where emergent pulse noise caused by engine revolution, wipers, or door mirrors, for example, become a problem, and noise cancellation techniques for removing such noise have been proposed and put into use.
Meanwhile, in the case of radio receivers, the level of background noise increases with the deterioration of the reception electric field. Although this noise is a white noise called weak-electric field noise, there is a problem that, during such time of electric field deterioration, a pulse noise canceller misoperates due to background noise. Consequently, a noise cancellation technique which includes an Automatic Gain Control (AGC) circuit and a noise detection circuit is proposed (see Patent Reference 1). FIG. 1, FIG. 2, and FIG. 3 show an outline of a conventional noise cancellation technique. FIG. 1 is a block diagram showing the configuration of a conventional noise canceller. FIG. 2(a) is a diagram showing the relationship of High-pass Filter (HPF) output and AGC output at the time when pulse noise is generated. FIG. 2(b) is a diagram showing the relationship of HPF output and AGC output at a time of electric field deterioration. FIG. 2(c) is a diagram showing the relationship of HPF output and AGC output at a time of electric field deterioration and when pulse noise is generated.
In FIG. 1, the conventional pulse noise canceller includes a delay device 100, a pulse detecting unit 200, and a pulse suppressing unit 300. An FM composite signal is inputted to the pulse detecting unit 200, and pulse noise is detected. At the same time, after the FM composite signal passes the delay device 100 for a detection delay amount, the pulse noise in the detection interval is removed by the pulse suppressing unit 300. The pulse detecting unit 200 is configured of a High-pass Filter (HPF) 201, an AGC circuit 202, and a comparator 203. The high-pass filter 201 passes only the high-frequency component of the FM composite signal. The AGC circuit 202 applies gain control depending on the output signal level of the high-pass filter 201. Specifically, as shown in FIG. 2(a), when the high-pass filter output is only pulse noise, the rectified signal level is low and thus AGC output gain remains high. In contrast, as shown in FIG. 2(b), when the background noise of the high-pass filter output increases, the rectified signal level becomes high and thus AGC output gain becomes low, and gain control is applied so as to suppress misdetection due to background noise. The pulse detection unit 200: compares the output level of the AGC circuit 202 and a predetermined threshold (the “second threshold” in the Claims); detects, as pulse noise, the case where the AGC output level exceeds the threshold; and controls the pulse suppressing unit 300. Here, as shown in FIG. 2(b), since even the pulse noise level is limited when gain control is performed by the AGC circuit 202 at a time of electric field deterioration, even pulse noise cannot be detected by using the initial threshold as-is. Consequently, as shown in FIG. 2(c), by controlling the threshold depending on the output gain of the AGC circuit 202, pulse noise can be detected without misdetection, even at the time of electric field deterioration. The pulse suppressing unit 300 can be implemented through an interpolation circuit that outputs an inputted signal as-is during a period in which a pulse is not detected and, during an interval in which a pulse is detected, holds the value immediately preceding the detection interval, for example.
Patent Reference 1: Japanese Patent No. 3213495